Porting to Windows RT from Windows Store (x86)

I have a game in Visual Studio (developed in Unity 3D Pro) that is deployed / built for x86 and currently in the Windows Store. If I want to make an ARM version for Windows RT devices, what exactly do I need to do? Do I need to build it using ARM setting in Visual Studio and submit it as a new produce for certification or can I change the buid settings to ALL Platforms and submit it as an updated release for my existing game?

Building for arm is not very different. The only (major) problem I can think of is the weak memory management where you could have some trouble if you are using variables from multiple threads without making them volatile.

I've more or less completed the port of my game to Windows Phone, which is obviously ARM. The only thing I've noticed is random crashes on one particular level. The game is very stable on x86 but I do get the occasional crash on the ARM build. I have Unity Pro so do have access to the profiler but the crashes are happening in the build and not within the editor so I'm not sure how to debug them. Still, they're not that common and could be related to the damaged phone I use for testing so I may just release and see what feedback I get and then work from there.